1. Field of the Invention
The present invention is broadly concerned with an improved two-step process for the preparation of polyesters wherein use is made of a first-stage catalyst having a relatively low decomposition temperature allowing substantially complete thermal decomposition of the catalyst prior to the second-stage reaction. More particularly, the invention pertains to such an improved process involving an initial catalyzed reaction between a carboxylic acid (e.g., isophthalic acid or terephthalic acid) and an alkylene oxide (e.g., propylene or ethylene oxide) in the presence of a low-degradation temperature catalyst such as benzyltriethylammonium chloride in order to form an oligoester mixture; in preferred practice, the reaction product is heated to essentially completely decompose the first-stage catalyst, whereupon the oligoester reaction product is reacted with a dibasic acid or anhydride. Advantageously, the first-stage catalyst has a thermal degradation temperature at or below the second-stage reaction temperature.
2. Description of the Prior Art
A well-known route for the production of polyester resins involves an initial reaction between a carboxylic acid and an alkylene oxide in the presence of a catalyst to form an oligoester (typically hydroxyalkyl ester) reaction product. This first-stage addition reaction is generally carried out at an elevated temperature on the order of 100.degree.-230.degree. C. and at a superatmospheric pressure of up to about 15 kg/cm.sup.2. A variety of catalysts have been proposed for use in this context, such as simple amines (U.S. Pat. No. 4,306,056) and alkyl quaternary amine compounds (U.S. Pat. No. 4,560,788).
After the first-stage reaction is completed, it is conventional to heat the reaction mixture in the presence of an inert gas such as nitrogen in order to remove the first stage catalyst. At this point, the catalyst-stripped first-stage reaction product may be mixed with a glycol such as dipropylene glycol and the mixture is reacted with a diacid or anhydride, especially maleic anhydride at elevated temperatures and pressures. This yields the desired unsaturated polyester product.
A significant problem with prior two-stage processes for polyester resin production stems from the fact that residual first-stage catalysts remaining in the second-stage reaction mixture tends to significantly discolor the final polyester resin products. Moreover, the attempted thermal degradation of the first-stage catalysts represents a material energy input to the process, thus raising costs.
There is accordingly a real and unsatisfied need in the art for an improved two-stage polyester resin process which ameliorates the problems of resin discoloration and excess energy usage.